Method for improving operational stability of electrocoating bath



G. E. F. BREWER ET AL 3,444,063 METHOD FOR IMPROVING OPERATIONALSTABILITY May 13, 1969 OF ELECTROCOATING BATH Filed Dec. 6, 1366 4 rfoe/v5 VS Sheet 2 of 2 G. E. F. BREWER ET AL METHOD FOR IMPROVINGOPERATIONAL STABILITY OF ELECTROCOATING BATH 650/965 5/? BRA/V6?6/185??? 4 BZ/AA S/fiE I N VENTORS BY Q4 25?" 04;, 5% 47'7'OR/VEVS May13, 1969 Filed Dec. 6, 1966 A M.Nv\\\ M NM Ni Qwi T N3 82 Q@ UnitedStates Patent 3,444,063 METHOD FOR IMPROVING OPERATIONAL STA- BILIZITYOF ELECTROCOATING BATH GeorgeEfF. Brewer, Novi, and Gilbert L. Burnside,Oak

Park, Mich., assignors to Ford Motor Company, Dearborn, Mich., acorporation of Delaware Filed Dec. 6, 1966, Ser. No. 599,438

Int. Cl. C23b 13/00 US. Cl. 204-181 10 Claims This invention relates tothe art of coating an electrically-conductive workpiece in an aqueousbath by electrically induced deposition of an organic coating materialhaving functional groups in its molecular structure which are ionizablein aqueous medium. More particularly, this invention is concerned with amethod for maintaining bath stability in a continuous or intermittentlycontinuous electrocoating process of the type herein described whichcomprises isolating the bath material in time or space from the coatingoperation and maintaining a difference of electrical potential betweenelectrodes in contact with the aqueous bath material that is below thethreshold voltage for the resin employed and sufiicient to effectremoval of ions that accumulate in the bath during coating operations.

The threshold voltage, i.e. the difference of potential at whichelectrically irreversible and/or bath insoluble deposition of the resinis initiated, will vary somewhat with the chemical and physicalproperties of the resin to be de posited. Ordinarily, this will be aboveabout 5 volts and less than about 20 volts, more commonly below aboutvolts. At voltages above the threshold voltage, an electricallyresistant, resin-comprising film forms upon the receiving electrode, theanode in anodic deposition, the current falls rapidly and coating iseffectively terminated as the current flow drops to insignificance.

The instant process is useful with either an anodic or a cathodicdeposition process. Resins suitable for anodic electrocating orelectropainting are polycarboxylic acid resins which are dispersed inthe aqueous coating bath with the aid of a water soluble amino compound,such term herein including water soluble amines and ammonia. A resinsuitable for cathodic deposition has in its molecular structureionizable groups which upon ionization leave ionically positive sites.Examples of such groups are amine and substituted amine groups such asquaternary ammonium groups. Dispersion of such resins is effected by theaddition of water-ionizable acidic dispersal assistants, e.g. watersoluble carboxylic acids and suitably buffered forms of certaininorganic acids. Formic acid and acetic acid are exemplary of the formerand buffered phosphoric acid is exemplary of the latter.

To date anodic deposition has gained primary acceptance in this fieldand is exemplified by US. Patent 3,23 0,- 162 to A. E. Gilchrist whichpatent is herein incorporated by reference. This embodiment of theelectrocoating process is used herein for the detailed description ofthe invention, it being understood that the polarity of the electrodesshown in the drawings are reversible for cathodic depositionembodiments.

During the coating process, the coating material is first deposited uponthe more easily coated areas, i.e. those areas of the workpiece aboutwhich the electric field is strongest. As these areas are coated with anelectrically resistant film, the coating material is then deposited uponthe uncovered areas and the areas having thinner coatings. Hence, thiscoating process tends to be self-leveling.

In industrial electrocoating processes, the coating tank may contain inexcess of 10,000 gallons of the aqueous bath and maintenance of bathquality through many turnovers, i.e. complete replacement of coatingsolids, is essential to economic operation. The initial conductivity ofa freshly prepared coating bath can be controlled within limits toprovide optimum coating conditions in the given system. However, inanodic deposition, the quantitative use of water soluble amines used tosolubilize the acidic resins affect such conductivity and hence the pHof the resultant bath. During continued use of the bath, an increase ofelectrical conductivity is frequently observed. In many instances, thisconductivity increase is accompanied by increased gas evolution at theanode, higher current consumption, lowered throwing power and formationof unsightly paint films upon curing. This increase in conductivity alsoresults from the accumulation of extraneous materials which ionize inthe bath. Such ions are introduced into the coating bath throughaddition of water, leaching of pigment, entrainment of salts withincoming workpieces, absorption of carbon dioxide from the air, etc. Bythese methods phosphate ions, sulfate ions, chloride ions and variousmetal ions are introduced into the bath.

It now has been discovered that bath stability, i.e. constancy of bathcomposition, is improved by placing the bath between electrodes ofopposite polarity and maintaining a difference of electrical potentialbetween such electrodes that is sufiicient to effect neutralization ofat least a portion of the offending ions at the receiving elec' trodewith resultant deposition or evolution of the reaction product. The termreceiving electrode refers to electrode of opposite polarity withrespect to the charge of a given ion. Such difference of potential mustbe maintained below the threshold voltage to prevent deposition of theresinous coating material upon the paint-receiving electrode, the anodein anodic deposition, with resultant loss of coating material andtermination of effective current flow. At the same time, positive ionsare deposited at the cathode.

The process can be carried out within the coating tank employing theelectrodes used in the coating process when the coating process is notin use or, preferably, a portion of the bath is continuously removed andpassed through a separate cell which can be operated continuously. Apotential difference of about 5 volts will be suitable with most baths.Depending upon the resin employed, a potential above about 1 and below 8volts, more commonly between about 2 and about 6 volts, will be foundadvantageous. It will be understood that the bath in use can be analyzedto determine the extraneous ions that have accumulated therein sinceinitiation of coating with the bath involved. A review of standarddeposition tables will provide information as to the depositionpotentials for the materials involved. Since the receiving electrode maybe of inexpensive sheet stock, it is advisable to replace this from timeto time. Likewise, the entire cell can be disconnected and cleansedperiodically.

The process to which the instant invention is directed together with themethod of this invention will be more fully understood from thefollowing detailed description when read in conjunction with theaccompanying schematic drawings, wherein:

FIGURE 1 is a schematic side view of apparatus which may be used forcarrying out the instant invention; and

FIGURE 2 is a schematic and partial top view of an alternative designfor a deionizing cell employed in conjunction with a conventionalelectrocoating tank.

Referring first to FIGURE 1, an electrically conductive tank 11 servesas the cathode of the coating cell when the coating process carried ontherein is one of anodic deposition. Tank 11 contains an aqueous coatingbath 13 in which a polycarboxylic acid resin and a water soluble amineare dispersed. Tank 11 is in electrical connection with ground andserves as the negative electrode in the coating process being connectedto a negative lead of a DC. power source 17 via conductor 15. Article 19which is to be electrocoated is placed upon conductor hangers 21 and 23which in turn are suspended from and transported through bath 13 by agrounded conveyor 35. Conveyor 35 is a conventional, electricallypowered, chain conveyor. Hangers 21 and 23 are electrically insulatedfrom conveyor 35 by insulators 25 and 27 to isolate article 19 from thegrounded conveyor. Contact plates or brushes 29 and 31 ride against andare shown in electrical connection with bus bar 33. Bus bar 33 iselectrically connected to a positive lead of power source 17 throughconductor 37.

An article 19 moves from right to left with respect to the drawing andenters bath 13, a difference of electrical potential is provided betweenarticle 19 and tank 11 of sufficient magnitude to initiateelectrodeposition of the dispersed resin in bath 13 upon article 19.This difference of potential may be as low as the threshold voltage forthe particular resin under the conditions of temperature, bathconductivity, etc., then existing or it may be as high as about 500volts or more if the film of coating material being laid down hassufficient strength to avoid rupture at such potential. Ordinarily, inindustrial production, the voltage employed will be above about 50volts, commonly in the range of about 100 to about 250 volts.

A conduit 39 provides means for withdrawing coating bath 13 from coatingtank 11 and introducing the same into a second and smaller tank 41.Conduit 43 and pump 45 provide means for returning aqueous bath material13-1 from tank 41 into coating tank 11. Coating tank 41 is in electricalconnection with power source 17 via conductor 47 and serves as thecathode of a second and separate cell which is used to cleanse the bath.A conductor 49 and an anode 51 positioned Within tank 41. Anode 51 hereestablishes electrical connection between power source 17 shown plaitedhas a large surface area relative to the quantity of bath material 131in which it is immersed. The ratio of anode surface area to bath volumein tank 41 is advantageously at least times greater, preferably at least100 times greater, than the corresponding ratio of the surface area ofanode 19 to volume of bath 13 in tank 11. Preferably, the distancebetween electrodes is as small as is feasible and the unit may take theform of two parallel sheet electrodes separated by a thin stream ofcoating bath electrolyte. Conventional control means, not shown, in orconnected with power source 17 and/ or condoctors 47 and 49 provide adifference of electrical potential between tank cathode 41 and anode 51that is independent of the potential difference between tank cathode 11and workpiece anode 19.

In practice, the bath material 13-1 in tank 43 is sufficiently isolatedfrom the flow of current between the electrodes 11 and 19 to avoidelectrodeposition of the coating material upon anode 51 as a result ofsuch current. This may be effected by separating the tanks by a distancesufficient to reduce to insignificance the effect of the electrodes ofthe main bath on the coating material in tank 41, the conduits 39 and 43may be constructed to provide effective electrical shielding betweenbaths, etc. Those skilled in this art will recognize electricalshielding as that phenomenon which occurs even within the coating bathand renders more difiicult the coating of relatively inaccessibleinternal surface areas of hollow workpieces. In the coating process,this difficulty is overcome by use of secondary electrodes inserted intosuch hollows and insulated from the workpiece anode, by utilizingcoating materials and/ or coating conditions which increase throwingpower of the system, i.e. the ability to reach and coat such areas withan electrically irreversible coating, etc.

Referring now to FIGURE 2, there is shown a portion of an electrocoatingtank 111 containing an aqueous coating bath 113 each of which may be thesame as or similar to tank 11 and bath 13 of FIGURE 1. Positionedalongside coating tank 111 is a deionizing unit 140 including a tank142, inlet conduits 144 and 150, outlet conduits 146 and 152 and loopconduits 148 and 154. Tank 142 comprises two electrode sides 142-1 and1422 which, as here shown, function as cathodes, and a U- shapednon-conductive member 142-3 of which only ends 142-31 and 14232 arevisible in this view. Member l423 also forms the bottom of tank 140 andis united with electrode sides 1421 and 142*2 in water-tight sealcompleting the tank. Positioned within tank is a centrally positionedelectrode 156 which, as here shown, functions as an anode. Positionedbetween electrode 156 and electrode 142-1 is a separator 158. Betweenelectrode 156 and electrode 1421 is a separator 160. The constructionand function of separators 158 and 160 are hereinafter described. Inaccordance with this embodiment, electrocoating bath is removed fromcoating bath 113 in tank 111 via conduit 144, enters tank 142, passesbetween electrode 142-1 and separator 158, exits and reenters viaconduit 154, passes between electrode 156 and separator 158, and isreturned to tank 111 via conduit 146. In similar fashion, coating bath113 is removed from tank 111 via conduit 150, enters tank 142, passesbetween electrode 142-2 and separator 160, exits and reenters viaconduit 148, passes between anode 156 and separator 160, and is returnedto tank 111 via conduit 150. It should be understood that one or more ofthe conduits of this unit may be operatively connected with conventionalpumping means, not shown, where necessary to effect passage of theliquid bath as aforedescribed.

Separators 158 and 160 should be sufiicient obstacles to liquid flow tosubstantially reduce mechanical mixing of the liquid on opposite sidesthereof and sufiiciently open or porous that ionic conductivity betweencenter electrode 156 and each of the outer electrodes 142-1 and 142-2 isnot significantly limited thereby. Separators 158 and 160 may be textilesheets, e.g. glass fiber cloth, porous plastic sheets, e.g.polypropylene, etc. The counterfiow principle here employed providescontact of the bath stream with the anode on return to coating tank andseparation to the degree feasible from the cathode at which amine and/or amine ion concentration is highest.

The coating material in the bath is periodically or continuouslyreplaced as it is used up with replacement of the components thereof,i.e. binder resin, pigments, etc., being made in accordance with theratio of their deposit. This ratio may vary considerably from the ratioof such components in the coating bath. Water is also added from time totime to maintain the paint solids level relatively constant, e.g. 5percent.

In this application, painting by electrodeposition is meant to includethe deposition of finely ground pigment and/or filler in the ionizableresin herein referred to as the binder, the deposition of binder withoutpigment and/ or filler or having very little of same, but which can betinted if desired, and the deposition of other water reducible surfacecoating compositions containing the binder which might be considered tobe broadly analogous to enamel, varnish, or lacquer bases, and thecoating material for such deposition is termed a paint. Thus, thebinder, which is converted to a water-resistant film by theelectrodeposition and ultimately converted to a durable film resistantto conventional service conditions by final curing, can be all orvirtually all that is to be deposited to form the film, or it can be avehicle for pigmentary and/ or mineral filler material or even otherresins on which it exerts the desired action for depositing the film.Suitable resins include but are not limited to those specifically listedin US. Patent 3,230,162 to A. E. Gilchrist. The preferred resins foranodic deposition have an acid number between about 30 and about 300 andan electrical equivalent weight between about 1,000 and about 20,000.The term electrical equivalent weight is employed herein to mean thatamount of resin or resin mixture that will deposit per Faraday ofelectrical energy input. The conditions, procedures, and calculationswhich can be employed to determine electrical equivalent weight are setforth in detail in the aforementioned US. Patent 3,230,162.

EXAMPLE 1 A five-gallon bath of eleetrodepositable automobile primerpaint was removed from an industrial production coating bath after thebath had undergone twenty turnovers. The aqueous bath had intimatelydispersed therein a polycarboxylic acid resin having an electricalequivalent weight of about 1,040 and an acid number of about 65,pigments and other inorganic additives conventional to electrocoatingpaints. This material was placed in a coating tank and test panels werecoated by anodic deposition as hereinbefore described with a potentialdifference between anode and cathode of 160 volts. The panels were curedby conventional bake oven procedure and upon examination were found tohave a relatively rough surface.

An anode having exposed surfaces measuring 72 square inches wassuspended in the bath with a cotton filter bag serving as a membranearound the anode. A potential difference of 5 volts and a constantcurrent flow of about 0.125 ampere was maintained between anode andcathode for 16 hours. A test panel was then inserted into the bath andcoated at 160 volts. The film was cured as before and a decrease inroughness of the coated surfaces was observed when compared with thepanels coated prior to the 5-volt treatment.

EXAMPLE 2 A bath purification cell is prepared in a tank cathode 6 feetlong, 6 inches deep and 0.75 inch in width. An anode in the form of ametal panel measuring 5.5 feet in length, 8 inches in height, and 0.040inch in thickness is supported so as to extend into the tank to a depthof about 5 inches and positioned an even distance from the correspondingsides of the tank. The tank is filled to a depth of 5 inches with anaqueous electrocoating bath. A first conduit introduces additionalquantities of the bath continuously from a larger electrocoating tankspaced apart from the aforedescribed cell and in which steel workpiecesare being painted by electrodeposition at an impressed voltage of 180volts. A second conduit provides means for continuously removing bathmaterial from the first mentioned tank and returning it to the largertank wherein the coating operation is in progress. In the smaller unit,a constant difference of potential of about 5 volts is maintainedbetweeen the tank cathode and the anode suspended therein.

EXAMPLE 3 The procedure of Example 2 is repeated employing a variety ofcommercially available, industrial electrocoating paints. Tests are runwith a potential difference of 2, 4, 6 and 8 volts between the anode andcathode in the bath purification cell. With some electrocoating paints,coating of the receiving electrode (anode) is found to reduce thecurrent flow with resulting decrease in the efficiency of the cell. Insuch instances, the potential difference is reduced to about 5 volts.With other formulations comprising polycarboxylic acid resin andpigment, operation at 8 volts does not result in significant depositionof resin upon the receiving electrode and continuous operation at suchvoltage can be maintained.

EXAMPLE 4 This invention is practiced within an industrialelectrocoating tank without removal of the bath therefrom to a separatecell by maintaining a difference of potential of about 6 volts betweenthe cathode tank and a metal sheet anode suspended therein duringperiods in which the bath is not in use.

The foregoing examples are solely for purposes of illustration andshould not be considered as limitations upon the true scope of theinvention as set forth in the appended claims.

We claim:

1. In a method for coating an electrically conductive object in anaqueous bath with an organic resin having ionized sites thereon andintimately dispersed within said bath, said method comprising immersingsaid object within said bath, utilizing said bath as the aqueouselectrolyte and said object as a first electrode of an electricalcircuit comprising said bath, said first electrode, and a sec- 0ndelectrode in contact with said bath and spaced apart from said object,providing a difference of electrical potential between said firstelectrode and said second electrode sufiicient to cause a direct currentof electrical energy through said bath and between said first electrodeand said second electrode having direction and sufficiency to effectelectrodeposition of a coating of said resin upon said object, andremoving the resultant coated object from said bath and wherein saidbath accumulates an increasing concentration of ionized inorganicmaterials with continued use, the improvement which comprises placing atleast a portion of said bath in contact with and between a cathode andan anode and maintaining a difference of electrical potential betweensaid cathode and said anode which is below the potential required toeffect electrodeposition of said resin upon either said anode or saidcathode and above that difference of electrical potential required toeffect removal from said bath of ionized inorganic material.

2. The method of claim 1 wherein said difference of electrical potentialbetween said anode and said cathode is above about 1 volt and below thepotential at which electrodeposition of said resin is essentiallyelectrically irreversible.

3. The method of claim 1 wherein said difference of electrical potentialbetween said anode and said cathode is in the range of about 2 to about8 volts.

4. The method of claim 1 wherein said difference of electrical potentialbetween said anode and said cathode is sufficient to cause evolution ofgas from said bath and insufficient to effect electrodeposition of saidresin.

5. The method of claim 1 wherein a portion of said bath is continuouslyremoved from said electrical circuit, passed between said anode and saidcathode while an electrically conductive object is being coated withinsaid electrical circuit, and returned to said electrical circuit.

6. In a method for coating an electrically conductive object in anaqueous bath situated within a coating tank and having a polycarboxylicacid resin and a water soluble amine intimately dispersed therein whichcomprises immersing said object within said bath, utilizing said bath asthe aqueous electrolyte and said object as the first and positiveelectrode of an electrical circuit comprising said bath, said firstelectrode, and a second electrode in contact with said bath, spacedapart from said first electrode, and negative relative to said firstelectrode providing a difference of electrical potential between saidfirst electrode and said second electrode, a direct current ofelectrical energy through said bath and between said first electrode andsaid second electrode, and electrodepositing a coating of said resinupon said first electrode and wherein said bath accumulates anincreasing concentration of ionized inorganic materials with continueduse, the improvement which comprises removing a portion of said bathfrom said coating tank, placing said portion of said bath between acathode and an anode externally positioned with reference to saidcoating tank, maintaining a difference of electrical potential betweensaid cathode and said anode which is below that required to effectelectrodeposition of an electrically resistant film of said resin uponsaid anode and above that required to effect removal from said portionof said bath of ionized inorganic material, and returning said portionof said bath to said coating tank.

7. A method in accordance with claim 6 wherein said resin has an acidnumber in the range of about 30 to about 300 and an electricalequivalent weight in the range of about 1,000 to about 20,000.

8. A method in accordance with claim 6 wherein said difference ofelectrical potential between said anode and said cathode is betweenabout 2 and about 6 volts.

9. A method in accordance with claim 5 wherein the difference ofelectrical potential between said first electrode and said secondelectrode is in the range of about to about 300 volts.

10. In a method for coating an electrically conductive object in anaqueous bath situated within a coating tank and having a polycarboxylicacid resin and a water soluble amine intimately dispersed therein whichcomprises immersing said object within said bath, utilizing said bath asthe aqueous electrolyte and said object as the first and positiveelectrode of an electrical circuit comprising said bath, said firstelectrode and a second electrode in contact with said bath, spaced apartfrom said first electrode, and negative to said first electrode,providing a difference of electrical potential between said firstelectrode and said second electrode in the range of about 50 to about500 volts with resultant direct current of electrical energy throughsaid bath and between said first electrode and said second electrode,electrodepositing a coating of said resin upon said first electrode, andremoving the resultant coated object from said bath and wherein saidbath is maintained at a pH at which said bath absorbs carbon dioxidefrom the atmosphere, the improvement which comprises removing a portionof said bath from said References Cited UNITED STATES PATENTS 3,355,37311/1967 Brewer et al 20418l 3,355,374 11/1967 Brewer et al. 204-181 JOHNH. MACK, Primary Examiner.

H. M. FLOURNOY, Assistant Examiner.

US. Cl. X.R. 204-431

1. IN A METHOD FOR COATING AN ELECTRICALLY CONDUCTIVE OBJECT IN ANAQUEOUS BATH WITH AN ORGANIC RESIN HAVING IONIZED SITES THEREON ANDINTIMATELY DISPERSED WITHIN SAID BATH, SAID METHOD COMPRISING IMMERSINGSAID OBJECT WITHIN SAID BATH, UTILIZING SAID BATH AS THE AQUEOUSELECTROLYTE AND SAID OBJECT AS A FIRST ELECTRODE OF AN ELECTRICALCIRCUIT COMPRISING SAID BATH, SAID FIRST ELECTRODE, AND A SECONDELECTRODE IN CONTACT WITH SAID BATH AND SPACED APART FROM SAID OBJECT,PROVIDING A DIFFERENCE OF ELECTRICAL POTENTIAL BETWEEN SAID FIRSTELECTRODE AND SAID SECOND ELECTRODE SUFFICIENT TO CAUSE A DIRECT CURRENTOF ELECTRICAL ENERGY THROUGH SAID BATH AND BETWEEN SAID FIRST ELECTRODEAND SAID SECOND ELECTRODE HAVING DIRECTION AND SUFFICIENCY TO EFFECTELECTRODEPOSITION OF A COATING OF SAID RESIN UPON SAID OBJECT, ANDREMOVING THE RESULTANT COATED OBJECT FROM SAID BATH AND WHEREIN SAIDBATH ACCUMULATES AN INCREASING CONCENTRATION OF IONIZED INORGANICMATERIALS WITH CONTINUED USE, THE IMPORVEMENT WHICH COMPRISES PLACING ATLEAST A PORTION OF SAID BATHIN CONTACT WITH AND BETWEEN A CATHODE AND ANANODE AND MAINTAINING A DIFFERENCE OF ELECTRICAL POTENTIAL BETWEEN SAIDCATHODE AND SAID ANODE WHICH IS BELOW THE POTENTIAL REQUIRED TO EFFECTELECTRODEPOSITION OF SAID RESIN UPON EITHER SAID ANODE OR SAID CATHODEAND ABOVE THAT DIFFERENCE OF ELECTRICAL POTENTIAL REQUIRED TO EFFECTREMOVAL FROM SAID BATH OF IONIZED INORGANIC MATERIAL.